Signature ‘low’ to help strengthen monsoon current

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Vinson Kurian

Thiruvananthapuram, June 22 A low pressure area has formed over east-central Arabian Sea, the India Meteorological Department (IMD) said on Monday, livening up the monsoon settings overnight.

The ‘low’ provides the best yet signature of a strengthening monsoon current over southwest Arabian Sea, says Dr Akhilesh Gupta, leading operational forecaster and Adviser to the Ministry of Science and Technology.


But the system has to become further strengthened to ‘excite’ the Bay of Bengal before the latter can join the party. The Arabian Sea and the Bay are required to be in sync for monsoon to drive into central India.

The Arabian Sea branch has to ‘connect’ with the Bay arm to set up the typical seasonal monsoon flow for rains to cover peninsular and central India.

On Monday, the Arabian Sea wind speeds clocked around 30-35 knots (55-65 km/hr). They would have to accelerate to at least 40 knots (74 km/hr) before they can touch off some activity in the Bay.

After June 25, the Arabian Sea is shown to feature a ‘core’ of winds speeding up to in excess of 40 knots, and this would be sufficient to carry the Bay winds too, on board.

Once the threshold speed is achieved, the Bay too, would be able to host a ‘low’ close to the Andhra Pradesh-Orissa coast by June 26, as projected by the European Centre for Medium-Range Forecasts.


This would also help unfold the east-west ‘shear zone’ of monsoon turbulence filled by opposing winds between three km and six km in the atmosphere. This is the height-level in the atmosphere where the monsoon is most active.

The ‘shear zone’ sets up the platform for rain-bearing systems to play around in, and is forecast to be in tact until July 2. The ‘zone’ would be on view from July 26, getting strengthened with each passing day.

So, the monsoon would be in good shape until July 2 during when seasonal rains would have covered the entire west coast and peninsular India, besides having waded into central and east India.

But the advance along the eastern flank would peter out over Bihar, beyond which monsoon-friendly easterlies from the Bay would not able to penetrate, Dr Gupta said.

Delhi would have wait for longer to see the rains arrive. The normal date of onset is June 29.

An interaction between the monsoon easterlies and a westerly trough coming in from the northwest border is still being projected, but the gains would be limited to eastern India.


Meanwhile, on Monday, the IMD said that the offshore trough extending from Konkan coast to Kerala coast would generate widespread rainfall along the west coast during the next three days.

Conditions are favourable for further advance of monsoon over more parts of Maharashtra and remaining parts of Karnataka during the next two to three days.

Severe heat wave conditions have been prevailing over east Uttar Pradesh, Bihar, Jharkhand, Orissa, Chhattisgarh, east Madhya Pradesh, Vidarbha, west Madhya Pradesh, west Uttar Pradesh, Delhi and Rajasthan.

The Regional Met Centre, Chennai, said in an update that rainfall occurred at most places over Kerala and at a few places over coastal Karnataka during the 24 hours ending Monday morning.

Isolated rainfall occurred over Tamil Nadu, Andhra Pradesh, interior Karnataka and Lakshadweep.


Rain or thundershowers have been forecast at many places over Kerala, coastal and south interior Karnataka and at a few places over north interior Karnataka and Lakshadweep.

Isolated rain or thundershowers are likely to occur over Tamil Nadu, Puducherry and Andhra Pradesh.

(This article was published in the Business Line print edition dated June 23, 2009)
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